1. Field of the Invention
The disclosures herein relate to a liquid discharge head unit and an image forming apparatus.
2. Description of the Related Art
An inkjet recording apparatus is generally known as a liquid discharge recording type image forming apparatus, such as a printer, a facsimile machine, or a plotter, or a combination of these functions, that includes a recording head formed, for example, of a liquid discharge head (liquid droplet discharge head) discharging liquid droplets. The liquid discharge recording type image forming apparatus is configured to discharge ink droplets from its recording head onto a transferred sheet, thereby forming an image on the transferred sheet. The formation of the image includes recording, printing, and imaging. There are two types of the liquid discharge image forming apparatus including: 1) a serial type image forming apparatus in which the recording head discharges ink droplets onto the transferred sheet for forming an image while traveling in a main-scanning direction; and 2) a line type image forming apparatus in which the recording head discharges ink droplets onto the transferred sheet for forming an image without traveling in any direction. Note that the transferred sheet is not limited to paper, but may be any media such as an overhead projector (OHP) film insofar as ink droplets or other liquid droplets are adhered to the media. Such media are also referred to as “recording media to be recorded”, “recording media”, “recording paper”, and “recording forms”.
Note that in this application, the “liquid discharge recording type image forming apparatus” indicates an image forming apparatus that forms an image onto media such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, and ceramics by discharging liquid onto such media. Note also that “forming an image” or “image formation” not only indicates providing an image having some kind of meanings onto the media such as characters and symbols, but also indicates an image without having any meanings such as patterns (i.e., by simply discharging ink droplets onto the media). Further, “ink” is not limited to those generally called “ink”, but may include those called “liquid” used as a generic name capable of forming an image, such as recording liquid, fixing liquid, and “liquid”. The ink of this application also includes a specimens for deoxyribonucleic acid (DNA), resist, patterning material, resin, and the like. Moreover, the “image” is not limited to the image applied to a two-dimensional object, but may include the image applied to a three-dimensional object and to the image formed of a molded object.
In the liquid discharge recording type image forming apparatus, if air bubbles are mixed into the liquid discharge head (i.e., recording head), the liquid discharge head may discharge liquid droplets in wrong directions or may discharge in an inappropriate fashion. Thus, it may be necessary to efficiently eliminate air bubbles present in ink supply paths.
Japanese Patent Application Publication No. 2009-126044 (hereinafter referred to as “Patent Document 1”) discloses an example of an air bubble eliminating technology for eliminating air bubbles from a head tank (synonymously used with a “sub-tank” or “buffer tank” insofar as the ink tank is integrally formed in a head). In the air bubble eliminating technology disclosed in Patent Document 1, a recording head is configured to include a first ink chamber having an inflow port to which ink is flown from an ink supply source; a second ink chamber to which ink is supplied from the first ink chamber; a discharge port configured to discharge ink from the second ink chamber, thereby performing a recording operation; a first discharge port configured to discharge a fluid from the first ink chamber; a second discharge port configured to discharge a fluid from the second ink chamber; and a liquid-air separation unit provided between the first ink chamber and the first discharge port, or between the second ink chamber and the second discharge port, and configured to regulate the discharge of the liquid, where a fluid resistance from the inflow port to the first discharge port is lower than resistance from the inflow port to the second discharge port.
Another example of the air bubble eliminating technology for eliminating air bubbles from the head tank is disclosed in Japanese Patent Application Publication No. 2002-086748 (hereinafter referred to as “Patent Document 2”). In the air bubble eliminating technology disclosed in Patent Document 2, a sub-tank is configured to include an ink chamber having an ink introducing part and an ink discharge part, the ink introducing part and the ink discharge part both arranged in an upper part of the ink chamber. The ink chamber includes a supplementary feed valve formed of a compression spring configured to shut off an ink introducing path by closely pressing a ball against a valve seat formed of an elastic member having the ink introducing path provided in the upper part of ink chamber and a sealing part formed of an elastic member having a constantly closed slit in the center of the sealing part provided in the ink discharge part of the ink chamber, where air bubbles mixed inside the ink chamber are appropriately discharged from the slit of the sealing part.
The line type image forming apparatus, for example, includes a recording head unit having liquid discharge head integrally having corresponding heads and tanks for supplying liquids to the heads (i.e., head-tanks) arranged in an entire width direction of a recording medium. In the line type image forming apparatus having such a configuration, ink supplied from main-tanks is distributed in the order of the corresponding sub-tanks and distributors (distributing members) to distribute the ink to the corresponding heads. Note that the head tanks include corresponding filtering members configured to filter impurities from the ink.
However, nozzles provided in each of the heads are extra finely made (e.g., the nozzle diameter of φ24 μm). Thus, if a dissolved oxygen rate of the ink is high, oxygen is gradually accumulated in the ink, which may eventually cause an ink discharge malfunction. If air bubbles are mixed in the ink within the ink supply path including the head tanks, the ink containing the air bubbles may be distributed without eliminating the air bubbles or the dissolved oxygen rate of the ink may be increased. As a result, the heads may not be able to discharge appropriate amounts of the liquids (ink) or may not be able to discharge the liquids (ink) at all. Accordingly, it may be necessary to eliminate such air bubbles from the ink.
As described above, if the air bubbles are mixed into the liquid (ink) inside the head tanks, the recoding heads may not be able to appropriately discharge the ink, resulting in liquid discharge malfunctioning.